The present invention relates to a laser structure for fabricating a plastic laser and achieving electrically-driven lasing action.
A number of conventional solid state lasers are based on inorganic semiconductors (e.g., GaAs), and typically are electrically-driven. These lasers use a recombination of injected electrons and holes in a laser cavity. Optical and electrical requirements for these lasers are easily met as the inorganic materials exhibit high carrier mobility and provide large refractive index changes with use of closely-related materials.
Semiconductor lasers using organic or polymeric materials and electrically-driven laser action have recently attracted a great deal of interest. Organic solid-state lasers have the potential to provide a compact low-cost laser source over a broad range of wavelengths throughout the visible spectrum. Organic lasers also influence research in other areas and have led to advances with both organic and inorganic semiconductor lasers, as described in A. Dodabalapur et al., xe2x80x9cOrganic Solid-State Lasers: Past and Future,xe2x80x9d SCIENCE Vol. 277 (Sep. 19, 1997), at pp. 1787-788, incorporated herein. Examples include the successful realization of distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers with dye-doped polymers and the widespread use of IDP-based DFB and DBR lasers. Such lasers exhibit superior single frequency operation and high-speed modulation characteristics, e.g., as compared with Fabry-Perot lasers.
The prospect of realizing electrically-driven laser action with organic or polymeric semiconductors arises from two principal sources: (i) the high degree of success achieved in improving the quantum efficiency and operating lifetime of light-emitting diodes (LEDs), and (ii) the relatively low threshold for photopumped laser action in some classes of organic or polymeric semiconductors. These conditions are required, but not sufficient, to provide an efficient organic laser. There are many problems, for example, with conventional diode structures (which form the basis for nearly all inorganic semiconductors) when applied to organic semiconductors. One problem is that charge carriers, which are polaronic in nature, create absorption bands within the bandgap. These absorption bands appear to quench the gain generated in the diodes. It is well known from studies on the optical properties of organic materials that charge carriers cause such sub-gab absorption. Also, recent investigations suggest that the losses created by the charge carriers are greater than or equal to the optical gain. Accordingly, the losses are too large in comparison with the optical gain to be neglected.
A structure for an electrically pumped organic laser is described in U.S. Pat. No. 5,881,089 issued to Berggren et al. on Mar. 9, 1999, xe2x80x9cArticle Comprising an Organic Laser,xe2x80x9d which is assigned to the present assignee and incorporated herein by reference. The ""089 patent describes an electrically-pumped organic laser with a structure that does not use conventional electrical pumping, i.e., pumping by injection of electrons and holes into the diode structure. Drawbacks with using a single cavity diode structure for an organic semiconductor include electrode absorption losses and tradeoffs between carrier transport properties and optical properties. With the device of the ""089 patent, electrical power is converted to stimulated optical emission in a unitary device structure; a first region converts an electrical current into incoherent photons, and then the incoherent photons are coupled into a second region that comprises the laser cavity.
As may be appreciated, those involved in the field of lasers and semiconductor devices continue to seek to develop new designs to improve device efficiency and performance and to allow for the use of new materials, such as plastics.
Summarily described, the invention embraces an article comprising a laser having a structure that is well-suited for achieving electrically-driven lasing action with organic and polymeric LEDs and photoexcited lasers. The laser structure comprises a light-emitting device, e.g., a light emitting diode (LED), a substrate disposed adjacent the LED, and a laser. The light-emitting device comprises a plurality of layers including two spaced-apart conductive layers and a light emissive layer therebetween, such that incoherent radiation of a first wavelength is emitted from the light emissive layer. The radiation is received by the substrate, transmitted within the body thereof to the laser, and then received by the laser. At least one of the LED and the substrate is configured such that the radiation is concentrated as it is guided within the body of the substrate to the laser. The laser then receives the concentrated light to generate a light beam with a relatively low threshold for the laser action.
In one embodiment, the concentration of light along the substrate is achieved with the substrate having a first and a second side, at least two different regions within the body of the substrate, and a light-reflecting interface proximal the junction of the two different regions. The substrate is oriented relative to the light-emitting device that a substantial amount of the electromagnetic radiation is received by one of the two regions at the first side of the substrate, internally reflected at the interface of the two regions, and concentrated within the body of the substrate to the second side. In another embodiment, the light is concentrated with the LED comprising a microcavity LED.